Réactivités aux alcalis du grès de Potsdam dans les bétons

Numerous old concrete structures showing signs of disintegration are found in the Beauharnois–Valleyfield area located to the southwest of Montreal.After a short examination of some of the structures, evidences of alkali–silica reactivity appear to be related to sandstone aggregates belonging to the Potsdam group. This rock type, although common in the state of New York and in the provinces of Quebec and Ontario, is only very rarely used as an aggregate owing to its hardness and abrasion. Nevertheless, when available from important excavation sites it has sometimes been used as an aggregate with ordinary alkali-rich cements.The products of the chemical reactions between the siliceous aggregates and the cement were studied with a polarizing microscope, a scanning electron microscope, an electron microprobe, and a thermobalance and differential thermoanalyser.During these studies superposed layers of silica gel of variable composition were found and secondary minerals were also identified. The Na/K ratio was found to increase in the more recent layers of silica gel suggesting that sodium could have been added within the structures as winter de-icing salts.The hypothesis is put forward that even if a low alkali cement is used with this Potsdam sandstone, alkali–silica reactivity could still occur in the presence of alkalies from external sources.